cmake: set target_include_directories on build interface

Consumers of non-installed build directory don't have to manually
set include dirs.

c++/src/CMakeLists.txt

 
-include_directories("${CMAKE_CURRENT_SOURCE_DIR}")
-
 # Tests ========================================================================
 
 if(BUILD_TESTING)

c++/src/capnp/CMakeLists.txt

@@ -58,7 +58,10 @@ set(capnp_schemas
 add_library(capnp ${capnp_sources})
 target_link_libraries(capnp kj)
 #make sure external consumers don't need to manually set the include dirs
-target_include_directories(capnp INTERFACE $<INSTALL_INTERFACE:include>)
+target_include_directories(capnp INTERFACE
+  $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/..>
+  $<INSTALL_INTERFACE:include>
+)
 install(TARGETS capnp ${INSTALL_TARGETS_DEFAULT_ARGS})
 install(FILES ${capnp_headers} ${capnp_schemas} DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/capnp")
 

c++/src/kj/CMakeLists.txt

@@ -63,7 +63,10 @@ endif()
 #make sure the lite flag propagates to all users (internal + external) of this library
 target_compile_definitions(kj PUBLIC ${CAPNP_LITE_FLAG})
 #make sure external consumers don't need to manually set the include dirs
-target_include_directories(kj INTERFACE $<INSTALL_INTERFACE:include>)
+target_include_directories(kj INTERFACE
+  $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/..>
+  $<INSTALL_INTERFACE:include>
+)
 install(TARGETS kj ${INSTALL_TARGETS_DEFAULT_ARGS})
 install(FILES ${kj_headers} DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/kj")
 install(FILES ${kj-parse_headers} DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/kj/parse")